WASHINGTON (Reuters) - U.S. drug regulators warned doctors and patients against using high doses of Forest Laboratories Inc's antidepressant Celexa or its generic versions because of heart concerns.

The widely used antidepressant had carried a label saying that some patients may require a dose of 60 milligrams a day. But the U.S. Food and Drug Administration said new studies showed that a dose higher than 40 mg a day can lead to abnormal heart rhythm, which can be fatal.

Celexa "should no longer be used at doses greater than 40 mg per day because it can cause abnormal changes in the electrical activity of the heart," the FDA said in a statement posted on its website on Wednesday.

People with underlying heart conditions or with low levels of potassium and magnesium in the blood are at a higher risk of developing abnormal changes in the heart's electrical activity, which can lead to abnormal heart rhythms, the FDA said.

Celexa, known chemically as citalopram hydrobromide, affects a message-carrying brain chemical called serotonin and can be used to reduce anxiety and obsessive-type disorders.

Celexa is approved to treat depression in adults, but not children. The drug label for Celexa and its generic versions has now been revised to recommend lower doses, the FDA said.

For Forest, Celexa once was a blockbuster treatment but it is now widely sold by generic drugmakers. The company now relies heavily on sales of Celexa's derivative Lexapro, its biggest product that is set to lose U.S. patent protection early next year.

Forest Laboratories was not immediately available for comment. The drugmaker's shares were down 0.5 percent at $33.26 in late morning trading on the New York Stock Exchange.

Last year, Forest was accused of marketing Celexa and Lexapro for unapproved use in children and paying pediatricians kickbacks. The company agreed to plead guilty and pay more than $313 million to resolve charges.